1. Field of the Invention
The present invention relates to a structure of a display device having a transistor. In particular, the present invention relates to a structure of an active matrix display device having a thin film transistor manufactured over an insulator such as glass or plastics. In addition, the present invention relates to an electronic device where a display portion thereof is formed by using such a display device.
2. Description of the Related Art
In recent years, a so-called self-luminous type display device having a pixel that is formed of a light-emitting element such as a light-emitting diode (LED) has been attracting attention. As a light-emitting element used for such a self-luminous type display device, an organic light-emitting diode (OLED) (also referred to as an organic EL element, an electro luminescence: EL element, and the like) has been drawing attention and used for an EL display (for example, an organic EL display or the like). Since a light-emitting element such as an OLED is a self-luminous type, it has advantages such as higher visibility of pixels than that of a liquid crystal display, and fast response without requiring a backlight. The luminance of a light-emitting element is controlled by a current value flowing through the light-emitting element.
In addition, in recent years, development of an active matrix display device where a light-emitting element and a transistor for controlling light emission of the light-emitting element are provided in each pixel has been advanced. In the active matrix display device, not only display in high resolution with a large screen, which is difficult in a passive matrix display device, is capable, but also a low power consumption operation that exceeds a passive matrix display device is realized, has high reliability, and practical use thereof has been expected.
As a driving method of a pixel in the active matrix display device, which is classified in accordance with a type of signal inputted into a pixel, a voltage input system and a current input system can be given. The former voltage input system is a system for controlling a luminance of a light-emitting element with a driving element by inputting a video signal (voltage), which is inputted into a pixel, into a gate electrode of the driving element. In addition, the latter current input system is a system for controlling a luminance of a light-emitting element by making a supplied signal current flow through the light-emitting element.
Here, an example of a pixel structure in each display device where a voltage input method and a current input method are applied, and a driving method thereof will be briefly explained with reference to FIGS. 48 and 49. Note that the explanation will be given by exemplifying an EL display device as a typical display device.
FIG. 48 is a diagram showing an example of a pixel structure in the display device where a voltage input system is applied (see Reference 1: Japanese Patent Application Laid-Open No. 2001-147659). A pixel shown in FIG. 48 includes a driving transistor 4801, a switching transistor 4802, a storage capacitor 4803, a signal line 4804, a scanning line 4805, first and second power supply lines 4806, 4807, and a light-emitting element 4808.
In this specification, a transistor being in an on state refers to a state in which a gate-source voltage of the transistor exceeds the threshold voltage of the transistor, and a source-drain current thereof flows. A transistor being in an off state refers to a state in which a gate-source voltage of the transistor is less than the threshold value of the transistor, and a source-drain current thereof does not flow.
When the switching transistor 4802 is turned on by changing the potential of the scanning line 4805, a video signal inputted into the signal line 4804 is inputted into a gate electrode of the driving transistor 4801. In accordance with the potential of the inputted video signal, the gate-source voltage of the driving transistor 4801 is determined; thus, the source-drain current of the driving transistor 4801 is determined. This current is supplied to the light-emitting element 4808; thus, the light-emitting element 4808 emits light.
In such a manner, a voltage input system refers to a system for supplying the gate-source voltage and the source-drain current of the driving transistor 4801 by the potential of the video signal and making the light-emitting element 4811 emit light with a luminance in accordance with this current.
As a semiconductor element for driving the light-emitting element, a poly-silicon (p-Si) transistor is used. However, the poly-silicon transistor is likely to cause variation in electronic characteristics of a threshold voltage, an on current, mobility, or the like due to a defect in a crystal grain boundary. Even in a case where the same video signal is inputted when there is variation in the characteristics of the driving transistor 4801 in each pixel in the pixel shown in FIG. 48, there is variation in a luminance of the light-emitting element 4808 because the amount of the drain current of the driving transistor 4801 in accordance with the video signal is different.
On the other hand, with a current input system, the amount of a current supplied to a light-emitting element can be controlled without depending on characteristics of a transistor.
FIG. 49 is a diagram showing an example of a pixel structure in a display device where a current input system is applied (see Reference 2: Japanese Patent Application Laid-Open No. 2004-163673). A pixel shown in FIG. 49 includes a driving transistor 4901, first to third switching transistors 4902 to 4904, a storage capacitor 4905, a signal line 4906, first and second scanning lines 4907, 4908, first and second power supply lines 4909, 4910, and a light-emitting element 4911. A current source circuit 4912 is arranged in each signal line (each column).
First, the first and second switching transistors 4902 and 4903 are turned on by changing the potential of the first scanning line 4907. At this time, a video signal current flowing through the signal line 4906 is referred to as Idata. Since the first and second switching transistors 4902 and 4903 are turned on, the driving transistor 4901 is in a state of diode connection. Since the video signal current Idata flows through the signal line 4906 at this time, current flows between both the electrodes of the storage capacitor 4905 and the electric charge is held in the storage capacitor 4905; thus, it begins to generate potential difference between both the electrodes. Then, gate potential of the driving transistor 4901 decreases; thus, current flows through a source from a drain. In the storage capacitor 4905, the storage of the electric charge is continued until the potential difference between both the electrodes, that is, the gate-source voltage of the driving transistor 4901 becomes a desired voltage. In other words, the storage of the electric charge is continued until an enough voltage for the driving transistor 4901 to make an Idata current flow is obtained. Meanwhile, when the storage of the electric charge is finished, current does not flow through the storage capacitor 4905; thus, an enough gate-source voltage for the driving transistor 4901 to make an Idata current flow is held in the storage capacitor 4905. Through the above operations, a writing operation of a signal to a pixel is completed. Lastly, with the selection of the first scanning line 4907 finished, the first and second switching transistors 4902 and 4903 are turned off.
Subsequently, the third switching transistors 4904 is turned on by changing the potential of the second scanning line 4908. Since the gate-source voltage, which is previously written, is held in the storage capacitor 4905, the driving transistor 4901 is turned on and a current equivalent to Idata current flows from the first power supply line 4909. Accordingly, the light-emitting element 4911 emits light. At this time, the light-emitting current flowing through the light-emitting element 4911 flows without any change even the source-drain voltage of the driving transistor 4901 is changed, as long as the driving transistor 4901 is made to operate in a saturation region.
In such a manner, a current input system refers to a system for supplying the drain current of the driving transistor 4901 so as to be the same current value as the video signal current Idata supplied in the current source circuit 4912 and making the light-emitting element 4911 emit light with a luminance in accordance with this drain current. By using the pixel of the above structure, a desired current can be applied to a light-emitting element by suppressing adverse effect of variation in characteristics of a transistor constituting a pixel.
However, in the pixel structure of the conventional current input system, it takes up much time to charge a parasitic capacitance or the like of a signal line by a video signal current Idata. In particular, in a case where it is desired to display a low gray scale, the video signal current Idata becomes extremely low; therefore, the charge time of a parasitic capacitance or the like of a signal line is not sufficient against a horizontal scanning period; thus, a video signal becomes incapable of writing accurately.
In addition, in each of the conventional pixel circuits (FIG. 48 and FIG. 49), the storage capacitor is connected between the gate and the source of the driving transistor. However, in a case of forming this storage capacitor with a MOS transistor, a channel region is not evoked in the MOS transistor when a gate-source voltage of the MOS transistor becomes almost equivalent to the threshold voltage of the MOS transistor; therefore, the MOS transistor does not serve as a storage capacitor. As a result, a video signal cannot be held accurately.
In such a manner, with the conventional voltage input system, variation in a luminance is caused due to variation in electronic characteristics of a transistor, whereas with the conventional current input system, particularly in a low gray scale display, the charge time of a parasitic capacitance or the like of a signal line is not sufficient; thus, a video signal cannot be written accurately.